1. Field of the Invention
The present invention relates to a niobium (Nb)-containing superconductor-laminated aromatic polyimide material. More particularly, the present invention relates to a flexible Nb-containing superconductor-laminated aromatic polyimide material in which an Nb-containing superconductor layer is formed on a substrate in the form of, for example, a film or filament, and comprises an aromatic imide polymer having an excellent heat resistance.
2. Description of the Related Art
Various highly efficient superconductors comprising a metal, alloy or metal compound and exhibiting a superconducting property at the temperature of liquid helium or higher are known, and those superconductors are utilized over a wide range of applications, for example, transportation systems, heavy electric machinery, computers, and medical machines, and further improvements thereof, and therefore an even wider range of utilization thereof, are expected.
Interest is now shown in Nb containing superconducting materials as superconductor materials having a high mechanical strength and a critical temperature of about 15 K higher than the liquifying temperature of helium. In particular, it has been found that NbN exhibits a high intensity of the super-conducting critical magnetic field Hc.sub.2 and little deterioration of the superconducting property thereof due to stress in a high magnetic field, and thus is expected to be especially useful as a superconducting magnetic material. Also, it was found that the NbN exhibits relatively little deterioration of the superconducting property thereof when exposed to irradiation by neutrons, and thus is expected to be useful as a magnetic material for nuclear fusion.
When used as a magnet or electron device in practice, the superconducting material comprising a metal, alloy or metal compound must be shaped into a wire form, tape form, thick film form or thin membrane form, and supported by a substrate (or support) in the form of a wire, tape, film or plate.
It is known that the substrate can comprise a shaped ceramic material in the form of a filament or plate made from, for example, ZrO, MgO, Al.sub.2 O.sub.3 or SiO.sub.2, and attempts have been made to provide superconducting materials usable for superconducting circuits and magnetic-shielding plate, in which a superconductor layer is formed on the conventional substrate. Nevertheless, the conventional substrates made from inorganic ceramic material are disadvantageous in that, when the substrate must be shaped by complicated shaping or forming procedures, the resultant substitrates are far too heavy and rigid and have an unsatisfactory mechanical strength.
Especially, when inorganic ceramic materials are shaped to form a light substrate or a thin membrane substrate, the resultant substrates have a significantly reduced mechanical strength.
Also, attempts have been made to produce superconductor composite wires by embedding a superconductor wire in a matrix comprising an Ag, Cu or a Cu-Sn alloy. Nevertheless, these attempts are not always successful in practical use, due to severe restrictions on combinations of the type of superconductor material with the type of matrix material, and an unsatisfactory forming property and weight of the resultant composite superconductor wire.
Accordingly, it is difficult to produce a composite superconducting material having a high flexibility, a low weight, and a high mechanical strength, in the form of a thin membrane or wire, by the conventional method in which an Nb-containing superconductor layer is formed on a conventional substrate.
Accordingly, there is a strong demand for the provision of a flexible and light weight substrate which can be easily shaped or formed, exhibits a satisfactory heat resistance, mechanical strength, and stability, even at a high temperature at which a superconductor layer is formed on the substrate, and has a good durability, flexibility and mechanical strength even at an extremely low temperature at which, for example, liquid nitrogen is stably maintained in the liquid state, and useful for producing practically useful superconducting materials.